This invention relates to multi-gas analysis in general, and specifically to multi-gas analysis of respiration by glow discharge emission spectrometry.
Respiratory monitoring allows analysis of physiological parameters including pulmonary blood flow, cardiac output, oxygen consumption, and anaerobic threshold. These physiological parameters are relevant for monitoring individuals in the fields of health care and athletic training, and especially during space flight where weightlessness dramatically affects these parameters.
Respiratory monitoring measures the above physiological parameters by analyzing the oxygen, nitrogen and carbon dioxide levels, among others, in the individual's respiration. The multi-gas analyzer commonly used for respiratory monitoring is the mass spectrometer which, however, is subject to random filament failure and requires a two-stage high vacuum system which can be troublesome during start-up. The measurement of all three of these gasses with the combination of three separate analyzers would lead to a number of difficulties, including a very bulky assembly, the reliability and maintenance of three separate analyzers, and the problem of matching three different response curves and lag times.
U.S. Pat. No. 3,951,607 and U.S. Pat. No. 3,996,010, both issued to Robert B. Fraser, disclose glow discharge emission spectrometers that function with a simple glow discharge transducer requiring only a moderate vacuum. The operation of the glow discharge emission spectrometer is based on the fact that when a gas sample is introduced into a low-pressure chamber in an electric field under the proper conditions of pressure, voltage, and chamber geometry, a glow discharge is formed. Selected spectral bands exist near the cathode region of this discharge (the negative glow) where the intensity of emission is proportional to the concentration of the component of interest in that gas sample. Monitoring the intensities of these bands provides a continuous measure of the component gas concentrations.
The glow discharge emission spectrometer of these two patents solve many of the above mentioned problems, however they require a large gas flow system (due to a high-flow vacuum control system) and a large, complex calibration system. Therefore, these spectrometers are too large and bulky for use in limited space environments, such as, for example, space craft.
Furthermore, the spectrometers of the above patents suffer from reduction of the transparency of the windows of the glow discharge chamber through which the negative glow radiation passes for spectral analysis. The sampled gas always contains oxygen. The high energy of the discharge causes the flow of oxygen ions to strike the cathode and produce an oxide of the cathode metal. This cathode oxide is sputtered off and deposited on the walls of the glow discharge chamber as well as on the windows.
The present invention provides a glow discharge chamber having a geometric configuration and anode and cathode orientation which minimizes the above cathode oxide sputtering. Furthermore, this glow discharge chamber allows calibration of the glow discharge emission spectrometer using fewer steps and few gasses than previously required, thus resulting in a less complex, smaller spectrometer. Specifically, calibration known in the art requires spectral analysis of seven different combinations of three gasses (O.sub.2, N.sub.2, and CO.sub.2) while the calibration method of the present invention employs only two different gas combinations employing O.sub.2, N.sub.2 and CO.sub.2.
Furthermore, the gas flow system of the present invention is a low-flow vacuum control system that reduces the overall size of the spectrometer.